Phase modulation schemes are very effective and are therefore widely used in communication systems. A simple example of a phase modulation scheme is quaternary phase shift keying (QPSK). FIG. 1 shows a constellation diagram that illustrates how QPSK maps two-bit digital data to one of four phase offsets. FIG. 2 shows a typical QPSK (or in-phase (I)/quadrature (Q)) modulator used to generate a phase-modulated signal. This technique relies on orthogonal signal vectors to realize the phase offsets—an inherently linear technique, since it depends solely on the matching of these orthogonal signals.
The I/Q modulator provides a straightforward approach to generating phase-modulated signals that is also suitable for more complex schemes such as wideband Code-Division Multiple Access (CDMA) and Orthogonal Frequency Division Multiplexing (OFDM) systems. It is also possible to generate the phase-modulated signals using a phase-locked loop (PLL). This approach offers reduced circuitry and lower power consumption and, as a result, finds widespread use in narrowband systems. A variation of this approach, known as two-point modulation, introduces direct modulation of the VCO to support wideband phase modulation, which unfortunately requires an accurate VCO gain. This requirement is a difficult task since the VCO gain depends on multiple factors. It would therefore be advantageous to accurately set the gain of the VCO.